The present invention relates generally to the fabrication of solid rocket propellants, and more specifically the invention pertains to a new technique for assessing the service life of a solid rocket propellant which allows one to characterize and analyze propellant behavior using very small samples which are nondestructively tested by dynamic mechanical measurements.
There is currently need for support devices capable of sensing the dynamic properties of propellant masses over long time periods without disturbing the integrity of the propellant charge. Should the properties of the propellant mass change significantly, then the operational capability of the rocket motor might be impaired.
Accelerated aging programs are a common method of evaluating the age stability of solid rocket propellants. Typically, 1/2-gallon cartons of propellant are aged at high temperatures, and during the aging period, the mechanical properties are measured by conducting tensile tests on slab samples cut from the cartons. The rate of change of a mechanical property parameter of interest (such as modulus) is then determined for each temperature. Usually an attempt is made to fit the aging rates to an Arrhenius model equation:
k=Aexp(-.DELTA.H/RT) (1)
where
k=aging rate constant (slope of the mechanical vs. time curve) PA1 .DELTA.H=activation energy PA1 R=gas constant PA1 T=absolute temperature PA1 U.S. Pat. No. 4,170,875 issued to Edwards; and PA1 U.S. Pat. No. 4,074,563 issued to Briar et al.
If the data fit such an equation (and a plot of log k vs. 1/T is linear), then the mechanical property degradation can be considered to be due to a single mechanism that is dominant up to the highest aging temperature. In this case, it is possible to calculate the change in the mechanical property parameter of interest under any aging condition. Thus, one can estimate the service life of the propellant for any temperature history. Of course, it is inadvisable to extrapolate the Arrhenius plot to higher temperatures than those used in the aging program.
In many cases, the aging data does not fit a simple Arrhenius model. If the log k vs. 1/T curve is not linear, then it is not possible to predict long-time ambient aging affects based on short-time thermally accelerated aging data. Such predictions are usually far too conservative, predicting much too short a service life for a system aged at ambient temperature.
The task Of assessing the service life of solid rocket propellant by nondestructive testing using dynamic mechanical measurement of very small samples conducted at small strain levels is alleviated, to some extent, by the systems disclosed in the following U.S. Patents, the disclosures of which are specifically incorporated herein by reference:
The Briar et al '563 patent discloses a dynamic sensor which is embedded within a propellant to sense and transmit information relating to dynamic modulus. The invention was devised to monitor the propellant mass as it stands, while encased within the rocket motor, so that a nondestructive assessment of the propellant properties might be determined. The Edwards '875 patent relates to a caseless rocket design which facilitates nondestructive testing.
While the above-cited references are instructive, a need remains to assess the service life of solid rocket propellants using motor samples as well as samples subjected to accelerated aging. The present invention is intended to satisfy that need.